FIG. 1 is a diagram illustrating a network configuration where multiple networks are connected. In FIG. 1, a subnet 1 includes terminals 1-1, 1-2, 1-3, 1-4, and 1-5 and Layer-2 switches (SWs) 2-1, 2-2, 2-3, 2-4, 2-5, and 2-6. Further, a subnet 2 includes terminals 1-6, 1-7, 1-8, 1-9, and 1-10 and Layer-2 switches (SWs) 2-7, 2-8, 2-9, 2-10, 2-11, and 2-12.
The subnet 1 and the subnet 2 are networks having IP address spaces independent of each other. The subnet 1 and the subnet 2 are connected by a network address translator (NAT) 3. The NAT 3 retains an address translation table in order to connect communications between a group of terminals in the subnet 1 and a group of terminals in the subnet 2. For example, the NAT 3 retains a pseudo IP address for the terminal 1-6. The pseudo IP address of the terminal 1-6 is defined in the IP address space of the subnet 1, to which the NAT 3 belongs.
It is assumed that the subnet 1 has a network address of 192.168.1.0/24, the subnet 2 has a network address of 192.168.100.0/24, the terminal 1-1 has an IP address of 192.168.1.1, and the terminal 1-6 has an IP address of 192.168.100.1. The NAT 3 has an address translation table in which a pseudo IP address of 192.168.100.11 for the terminal 1-1 and a pseudo IP address of 192.168.1.11 for the terminal 1-6 are recorded. When the terminal 1-1 transmits a packet to the terminal 1-6, the terminal 1-1 transmits the packet to the pseudo IP address of the terminal 1-6, 192.168.1.11, and the NAT 3 temporarily receives the packet. Then, the NAT 3 converts the destination IP address to 192.168.100.1 and converts the source IP address to 192.168.100.11, and thereafter transmits the received packet to the terminal 1-6.
In the case of connecting two networks by a single NAT as illustrated in FIG. 1, there is a problem in that an increase in traffic to be relayed causes an increase in a relaying load on the NAT. Therefore, the technique of connecting networks by multiple NATs as illustrated in FIG. 2 is provided.
In FIG. 2, the subnet 1 and the subnet 2 are connected by the NAT 3 and a NAT 4. In this case, the NAT 3 has an address translation table illustrated in FIG. 3A and the NAT 4 has an address translation table illustrated in FIG. 3B, so that packets addressed to the terminals 1-1, 1-2, 1-6, 1-7, and 1-8 are forwarded by the NAT 3, and packets addressed to the terminals 1-3, 1-4, 1-5, 1-9, and 1-10 are forwarded by the NAT 4. FIG. 3C illustrates a source address translation table in the NAT 3 and the NAT 4.
The technique is known of dividing an address range between multiple address management devices provided in a single network and causing the address management devices to be in charge of managing respective address ranges different from each other. (See, for example, Japanese Laid-open Patent Publication No. 8-186569.)
Further, the technique is known of connecting networks having different network addresses by a relay and causing the relay to respond by proxy to an ARP (Address Resolution Protocol) request from one to the other of the networks. (See, for example, Japanese Laid-open Patent Publication No. 2005-33302.)
Further, the technique is known of providing an active and a backup (standby) inter-LAN connection device on the same route. (See, for example, Japanese Laid-open Patent Publication No. 8-256173.)